diff --git a/internal/check-legend-export-reachable.mjs b/internal/check-legend-export-reachable.mjs
new file mode 100644
index 000000000..1330ae50a
--- /dev/null
+++ b/internal/check-legend-export-reachable.mjs
@@ -0,0 +1,138 @@
+#!/usr/bin/env node
+// check-legend-export-reachable — the Export affordance must be reachable AT BOTH WIDTHS.
+//
+// ★ WHAT THIS CAUGHT. The Export menu lives in SemanticTreeLegend, which upg-100-list.vue mounts
+// for DESKTOP only ("Hidden below the mobile breakpoint, where <LegendMobile>..."). LegendMobile —
+// the phone bottom-sheet — contains the string "export" ZERO times. So on a phone a reader can
+// browse the UPG-100 lists and cannot export a packet or a poster. No error, nothing to click; the
+// failure is invisible rather than broken, which is why it survived.
+//
+// The packet IS the product for this profile. The Driver: "print options at the top right of the
+// legend... that's the only way that map would be usable."
+//
+// ⚠ THE CONTROL IS GATED ON A SELECTION. `v-if="SHOW_EXPORT_BUTTON && exportEnabled && selectedNode"`
+// — so a probe that merely loads the page finds nothing at EITHER width and concludes "no export
+// anywhere". Measured: that was my first reading and it was wrong. This gate SELECTS A LEAF ROW
+// first, and asserts the 0 -> n transition, which is also what proves the control is selection-gated
+// rather than always-on.
+//
+// ⚠ AND ON MOBILE THE LEGEND STARTS CLOSED. 20 rows exist with 0 visible until the "Show or hide
+// the legend" control is clicked. A probe that skips that step reports "no rows on mobile", which is
+// a true statement about the wrong surface — the second wrong reading this gate encodes against.
+//
+// ⚠ VISIBILITY IS GEOMETRY, NOT A CSS PROPERTY. The mobile elements report display:block and
+// visibility:visible while measuring 0x0 at (0,0) inside an overflow:hidden parent — the desktop
+// panel rendered but never laid out. A predicate reading `visibility` would call that visible.
+// Assert a real box.
+//
+// Usage: node internal/check-legend-export-reachable.mjs [leafUrl]
+//   SKIPs when nothing is served — this needs the built tree.
+import { chromium } from 'playwright'
+
+const DEFAULT_LEAF = (process.env.BASE_URL || 'http://127.0.0.1:4173')
+  + '/doxa-maps/my-upg-100-list/upg-100-list/index.html'
+const LEAF = process.argv[2] || DEFAULT_LEAF
+
+let reachable = false
+try {
+  const r = await fetch(LEAF, { signal: AbortSignal.timeout(4000) })
+  reachable = r.ok
+} catch { /* a precondition, not a product failure */ }
+if (!reachable) {
+  console.log(`LEGEND-EXPORT-REACHABLE: SKIP — nothing served at ${LEAF}.`)
+  console.log('  Serve the built tree: python3 -m http.server 4173 --directory public (or set BASE_URL)')
+  process.exit(0)
+}
+
+const WALK = `(root, out) => { out = out || []; const rec = (r) => { for (const e of r.querySelectorAll('*')) { out.push(e); if (e.shadowRoot) rec(e.shadowRoot) } }; rec(root); return out }`
+
+async function probe(browser, { name, width, height, mobile }) {
+  const page = await browser.newPage({ viewport: { width, height }, isMobile: mobile, hasTouch: mobile })
+  const errs = []
+  page.on('pageerror', (e) => errs.push(String(e).slice(0, 110)))
+  await page.goto(LEAF, { waitUntil: 'load', timeout: 45000 })
+  await page.waitForTimeout(9000)
+
+  const before = await page.evaluate(`(${WALK})(document).filter(e => (e.className||'').toString().includes('stl-export')).length`)
+
+  // On a phone the legend starts closed — open it first, or every row query returns nothing.
+  const openedSheet = await page.evaluate(`(() => {
+    const all = (${WALK})(document)
+    const b = all.filter(e => e.tagName === 'BUTTON')
+      .find(e => /show or hide the legend|toggle.?legend/i.test(e.getAttribute('aria-label') || ''))
+    if (!b) return false
+    b.click(); return true
+  })()`)
+  if (openedSheet) await page.waitForTimeout(2200)
+
+  // Select a LEAF row. Height-bounded because a container also carries .stl-row-ish classes and
+  // clicking it selects nothing — measured: its textContent is the whole list concatenated.
+  const clicked = await page.evaluate(`(() => {
+    const rows = (${WALK})(document)
+      .filter(e => (e.className||'').toString().includes('stl-row'))
+      .filter(e => { const r = e.getBoundingClientRect(); return r.width > 0 && r.height > 0 && r.height < 60 })
+    if (!rows.length) return null
+    rows[0].click()
+    return (rows[0].textContent || '').trim().replace(/\\s+/g, ' ').slice(0, 30)
+  })()`)
+  await page.waitForTimeout(2200)
+
+  const after = await page.evaluate(`(() => {
+    const el = (${WALK})(document).filter(e => (e.className||'').toString().includes('stl-export'))
+    // A REAL BOX, not a CSS property: the mobile case reports visibility:visible at 0x0.
+    const boxed = el.filter(e => { const r = e.getBoundingClientRect(); return r.width > 2 && r.height > 2 })
+    return { total: el.length, boxed: boxed.length,
+             label: boxed.map(e => (e.textContent || '').trim().slice(0, 18))[0] || '' }
+  })()`)
+  await page.close()
+  return { name, before, openedSheet, clicked, ...after, errs }
+}
+
+const browser = await chromium.launch({ args: ['--use-gl=swiftshader', '--enable-unsafe-swiftshader'] })
+let results
+try {
+  results = [
+    await probe(browser, { name: 'desktop', width: 1500, height: 1000, mobile: false }),
+    await probe(browser, { name: 'mobile', width: 390, height: 844, mobile: true }),
+  ]
+} finally {
+  await browser.close()
+}
+
+console.log('LEGEND-EXPORT-REACHABLE')
+console.log(`  leaf     : ${LEAF}`)
+for (const r of results) {
+  console.log(`  ${r.name.padEnd(8)}: selected ${JSON.stringify(r.clicked)} — `
+    + `.stl-export ${r.before} -> ${r.total} present, ${r.boxed} with a real box`
+    + `${r.label ? ` ("${r.label}")` : ''}${r.openedSheet ? '  [opened the sheet first]' : ''}`)
+}
+
+const fails = []
+for (const r of results) {
+  if (!r.clicked) {
+    fails.push(`${r.name}: no selectable legend row — the probe could not reach the state the `
+      + 'export control is gated on, so this width was not actually tested')
+    continue
+  }
+  if (r.boxed === 0) {
+    fails.push(`${r.name}: the Export affordance has NO reachable box`
+      + (r.total > 0
+        ? ` — ${r.total} .stl-export element(s) exist but all measure 0x0, i.e. the desktop legend `
+          + 'rendered without being laid out. A reader here can browse the lists and cannot export '
+          + 'a packet or a poster.'
+        : ' — no .stl-export element at all.'))
+  }
+  if (r.errs.length) fails.push(`${r.name}: ${r.errs.length} page error(s): ${r.errs[0]}`)
+}
+
+if (fails.length) {
+  console.error(`\nLEGEND-EXPORT-REACHABLE: FAIL — ${fails.length} problem(s)`)
+  for (const f of fails) console.error('  - ' + f)
+  console.error('\n  The packet IS the product for this profile. See '
+    + 'BUG-UPG100-PACKET-AND-POSTER-EXPORT-ARE-UNREACHABLE-ON-A-PHONE — WHERE the control goes on a '
+    + 'phone is a design call (the sheet has no titlebar), so this gate names the gap rather than '
+    + 'assuming a fix.')
+  process.exit(1)
+}
+console.log('\nLEGEND-EXPORT-REACHABLE: PASS — selecting a list reveals a reachable Export control at '
+  + 'every width tested.')
diff --git a/package.json b/package.json
index 29e74aaee..ce0914f04 100644
--- a/package.json
+++ b/package.json
@@ -67,6 +67,7 @@
     "check:locale-reachable": "node internal/check-locale-reachable.mjs",
     "check:poster-prints": "node internal/check-poster-prints.mjs",
     "check:provider-switch": "node internal/check-provider-switch.mjs   # the provider switch must actually SWITCH: counts requests BY HOST (not window globals, which stay defined after a swap) and asserts the maplibre run stays within the leaf HTML's own declared api.mapbox.com preloads. Defaults to the upg-100 leaf; pass a URL or set BASE_URL for another. CONDITIONAL: SKIPs when nothing is served.",
+    "check:legend-export-reachable": "node internal/check-legend-export-reachable.mjs   # the Export affordance must be reachable at BOTH widths. Selects a leaf row first (the control is gated on selectedNode) and opens the mobile sheet first (it starts closed). Asserts a REAL BOX, not CSS visibility — on mobile the elements report visibility:visible at 0x0. ⚠ CURRENTLY FAILS ON MOBILE BY DESIGN: the export lives in the desktop legend only, and WHERE it goes on a phone is a Driver call (BUG-UPG100-PACKET-AND-POSTER-EXPORT-ARE-UNREACHABLE-ON-A-PHONE). Not wave-wired until then. SKIPs when nothing is served.",
     "check:mfe-two-tab": "node internal/check-mfe-two-tab.mjs",
     "check:share-policy": "node internal/check-share-policy.mjs",
     "check:zone-census": "node internal/check-zone-census.mjs   # every zone a USER CAN SELECT must have a poster (asks the LEGEND, not clusters.json — two collapsed deaf lists are unreachable and not a gap). CONDITIONAL: SKIPs without out/posters/, which is gitignored — run `bun run posters` first. It cannot be fixtured like check:packets-all, because it asserts COVERAGE of real generated output and a fixture would only assert the fixture is complete.",